This invention is directed to a process of powder coating of workpieces in which initially a workpiece is coated with powder so that the powder adheres to the workpiece and subsequently the powder is baked in an oven.
The complete, homogeneous (single-color) coating of workpieces with color powder for instance in electrostatic powder coating plants with subsequent baking of the color powder which adheres electrostatically to the workpiece, has been known (DE 3429580 A1, GB 1465146). When it was desired to form a multi-colored pattern on a workpiece the workpiece was covered with an appropriate mask, the respective color powder was applied to the exposed surface area of the workpiece to be coated with said color and subsequently baked by placing the entire workpiece in the baking oven. These steps were then repeated for every further color with corresponding covering masks and color powders until the desired pattern had been fully applied. Apart from the considerable time and energy requirements of said process it also exhibits the particular drawback that the quality of the coating is deteriorated by the multiplicity of baking steps.
EP 0,480,828 and EP 0,481,869 disclose processes of powder coating of workpieces in which a mixture of powder and a neutral gas is injected via a nozzle into a laser beam so that the powder either impinges on a workpiece together with the laser beam or initially passes through the laser beam and thereafter impinges on the workpiece. During this process the powder is melted by the thermal radiation of the laser, and the melted powder is deposited on the workpiece. Alternatively, the powder may also be applied to the workpiece prior to the laser treatment. In said coating process the velocity of the powder stream has to be relatively low so that the laser beam can reliably melt or fuse the powder particles. The relatively high temperatures produced in the process require the use of the neutral gas. Since the laser beam has a limited width such a coating process requires not only much technical effort but is also slow and only suited for relatively small areas.
This invention is based on the objective of providing a process of powder coating of workpieces in which locally exactly defined surface areas of a workpiece can be coated in a simple way. It is a secondary objective of this invention to provide a process in which multi-colored two-dimensional bodies such as images or patterns can be applied onto a workpiece by powder coating without having to subject the workpiece to a plurality of baking operations.
The specified objective is achieved in accordance with the invention by a process wherein powder adhering to the workpiece after coating and prior to baking is irradiated with electromagnetic radiation within a defined surface portion such that the powder present within the defined surface portion will gelate and stick to the workpiece. The color powder which adheres to the workpiece outside of the defined surface portion is removed subsequent to the electromagnetic irradiation step. In the process of the invention the workpiece is initially surface-coated with powder in the usual way, for instance in an electrostatic powder coating plant. Thereupon a desired defined surface portion of the workpiece is irradiated with electromagnetic radiation such as IR-light or UV-light so as to transfer energy to the powder and to initiate crosslinking so that the powder adhering to the workpiece will gelate in said defined surface portion and stick to the workpiece. The color powder adhering to the workpiece outside of the defined surface portion is removed such as by blowing. Subsequently, the color powder is baked in a baking oven. The process according to the invention eliminates the making and using of masks for desired patterns, while a two-dimensional pattern can easily and rapidly be applied to a workpiece surface.
For applying a multi-colored pattern or image to the workpiece surface, the workpiece is successively coated with a respective one of the color powders and irradiated with electromagnetic radiation within the defined surface portion allotted to the respective color. Thereafter any excess color powder outside of the associated surface portion is blown off, and the steps of coating, irradiating and blowing are repeated for each additional color of the workpiece until the desired pattern or image has been formed thereon. Subsequently, the entire color powder is baked in a single operation. With the method of the present invention it is therefore possible to apply a multicolored pattern or image in its entirety to the workpiece, the powder respectively gelling in the individual portions due to the electromagnetic irradiation, and thereafter to bake the pattern or image in a single operation so that the initially coated portions will not deteriorate due to multi-baking of the color powder.
Gelling here means first of all fusing of the powder. To this end, external energy must be transferred to the powder because an endothermal operation will take place. By the use of a laser, for example, the emission wavelength of the source of radiation can be adjusted accurately to the absorption wavelength of the powder so as to enable a highly effective energy transfer. As soon as the powder has changed to the liquid phase, so-called crosslinking will take place, i.e., three-dimensional molecular networks will be formed which finally constitute the homogeneous powder film which is what a powder coat actually is.
In addition to sources of IR-radiation also sources of UV-radiation are suitable for gelling or partial crosslinking of the powder. The important factor when selecting the source of radiation and the applied electromagnetic radiation is that the transferred energy should be able to initiate crosslinking in the powder or fusion of the powder.
The source of IR-radiation may, for example, be an IR-laser or a "Nernst stylus" which sweeps or "scans" the desired, defined surface portion or directly projects the desired pattern on the workpiece.
When an IR-laser is used as the source of IR-radiation it may be coupled directly with a computer for driving the laser or laser scanner so as to transfer a digitised image directly onto the workpiece.
As the source of radiation for partially gelling or partially crosslinking the powder, one may also make use of a source of UV-radiation of the kind used in stereolithography for producing three-dimensional workpiece patterns from liquid synthetic plastics. The chemical crosslinking which occurs in said process is very similar to the powder crosslinking process.
In view of presently feasible technologies and with suitable color powder there is provided electromagnetic irradiation to cause the color powder to gelate, the surface being heated for about 5 seconds at approximately 160.degree. C. The source of radiation is not limited to a particular spectral range. Rather, it is essential that the radiation emitted by the source be adjusted to the absorption occurring within the powder which will initiate crosslinking in the powder.